Liquid Crystal Displays (LCDs) are receiving increasingly wide use for advantages such as low radiation, low power consumption as well as slim and light construction, and have become the predominant displays at present.
In a liquid crystal display, due to the fact that the liquid crystal per se does not emit light, a backlight module is necessary for cooperating therewith to provide the display light source. One of the essential components for the backlight module is a light guide plate, which is used for converting a plurality of point or linear light sources into the form of an area light source, and guiding the emitted light to a display panel. By means of micro-structure formation processes such as injection molding, hot pressing and ink jet, pre-designed mesh structures are formed on the light guide plate, which is key to implementing a backlight module with outstanding optical luminance and homogenous style. In the mesh design for the light guide plate, a key parameter is the ratio between a distance, which is between the light source (e.g. light emitting diodes (LEDs)) and an active area of the display panel, and a pitch between the plurality of light sources, i.e. an A/P ratio. When A/P is relatively small, it is difficult to mix the light between the LEDs, so that hotspot phenomenon is very easily aroused. Specifically, FIG. 1 shows typically a light source part of an LED backlight module being one LED light bar, wherein there is a certain distance between the LEDs. Since the LEDs emit light in a certain angle, as the LED light bar is illuminated after being assembled with the light guide plate, there will be areas between the LEDs where light rays cannot arrive or the arrival light rays are considerably weak, giving rise to an optical phenomenon with alternating bright and dark areas, which is called “hotspot”.
As the liquid crystal displays develop towards a lighter and thinner profile, a narrower frame (reduced distance between the light source and the active area) and a lower power consumption (reduced number of light sources, increased pitch between the plurality of light sources), the A/P ratio will become smaller and smaller. When the A/P ratio is smaller than a predetermined value, even though the mesh structures on the light guide plate may be optimized, still dark areas will appear inevitably on the light guide plate thus leading to the phenomenon of hotspot. To take a display panel suitable for a notebook computer for example, it has been found that when the A/P ratio is smaller than 0.65, the light guide plate will be present with the hotspot that is non-improvable from the mesh design.
In order to solve the above problems, solutions in the prior art conventionally include: (1) attaching a black or white shielding tape at the U-turn of a back plate located below the light guide plate such that it is able to absorb or reflect intense light emitted from the light sources and shield as far as possible the light emitting areas to alleviate the hotspot; (2) forming a “V” shaped zigzag structure at an end face of the incidence side of the light guide plate by means of blade rotating machinery processing (V-cut process) so that the incident light is scattered to thereby alleviate the hotspot. Due to the introduction of a shielding tape, solution (1) of the above increases the difficulty in assembling and sacrifices the area of the light emitting regions, so that a side frame of the light bar becomes wider. Meanwhile, the black shielding tape causes loss of the emitted light from the backlight module for being absorptive to intense light. Due to the introduction of V-cut process, solution (2) of the above causes a decrease in the yield of the light guide plate and an increase in their costs, from which also debris from the light guide plate is easily resulted, and the light guide plate debris may scratch the LEDs (resulting in discoloration) and the light guide plate (resulting in bright spot).
On the other hand, the immense heat released along the light emission from LEDs greatly affects the light emission efficiency and the lifetime of the LEDs. It has been found that when temperature of the LEDs exceeds a predetermined value, the light emission efficiency and the lifetime of the LEDs will decrease in progression exponentially. According to the Arrhenius law, every 10° C. elevation of the temperature can result in a shortening by one half of the LED lifetime. Furthermore, the liquid crystal display in longtime operation can very easily cause an excessive concentration of heat surrounding the LEDs such that the LEDs and their circumferential circuitries are influenced disadvantageously.
At present, heat dispersion for LEDs depends largely on the use of an L/B laminated aluminum substrate, with which concentration of heat in spots (around the PN junctions of the LEDs) is split into a bar-like form of concentration to increase the areas of heat distribution and to implement heat dispersion by means of air convection. By doing so, although the over-heating can be alleviated, the air with an extremely low heat transfer coefficient may not transfer the heat efficiently. With respect to the LEDs in longtime operation, heat accumulation still persists since the heat generation rate is greater than the heat transfer rate.
CN 200810222740.4 describes a backlight lamp assembly, which comprises a circuit board and LEDs arranged on the circuit board; wherein, at a side of the circuit board arranged with the LEDs a thermoluminescent material layer is provided, and the thermoluminescent material layer is provided thereon with through-holes that correspond to the LEDs, said LEDs penetrating said through-holes.